1. Field of the Invention
This invention relates to improvements in an optical instrument having a zoom lens structure provided with a plurality of guide shafts for axially guiding a zooming optical system including a magnifying system and a correcting system. More specifically, the invention relates to an optical instrument having a zoom lens structure wherein openings for passing an object light beam therethrough are formed centrally of a frame member holding the opposite ends of the guide shafts for guiding the axial movement of the zoom lens. A beam splitter for splitting the object light beam into a picture-taking light beam and a viewfinder light beam is provided between the zooming optical system and the rear end wall of the frame member, and an opening for allowing the viewfinder light beam to pass is formed in the side wall of the zoom lens structure.
2. Description of the Prior Art
In the zooming optical system of the prior art, a most popular zooming operation is that known as the mechanical compensation type zooming which comprises moving both the magnifying optical system and the correcting optical system along the optical axis while maintaining them in a predetermined relationship. To allow such zooming operation, members for carrying the magnifying and correcting systems are movably mounted on axial guide members, whose opposite ends are held by frame members. The members for carrying the two axially movable optical systems and the guide members are usually fitted one within another, and a zooming operation ring forming a part of the zoom lens structure may be operated to move the two optical systems axially along the guide members. In such zoom lens structure according to the prior art, the rear frame member is formed with an opening for passage of the object light beam. For the TTL photography, the light beam passed through such opening is split into a picture-taking light beam and a viewfinder light beam by a beam splitter provided exteriorly of the zoom lens structure. Thus, in the zoom lens structure of the prior art, means such as a beam splitter or mirror for providing a viewfinder light beam must be provided rearwardly of the structure. This in turn has given rise to the necessity of reducing the axial length of the zoom lens structure itself as much as possible to thereby reduce the length over which the zooming optical system is projected beyond the body of a motion picture camera or like optical instrument. As a result, the lengths over which the holder members for the two movable optical systems and the guide members are fitted together have been so much limited that sufficient fitting precision could not be obtained. Such inferior fitting precision has caused inclination of the lenses with respect to the optical axis, which in turn has resulted in a lower accuracy of image formation on the focal plane.